Electric motors commonly include a stationary component called a stator and a rotating component called a rotor. The stator has a number of windings, and the rotor rotates within (or around) the stator when the stator windings are energized with a driving waveform. Electric motors are used in a variety of applications including HVAC systems, appliances, etc.
Electric motors sometimes fail prematurely. Motor failures interrupt the operation of the systems in which they are used and can cause significant economic losses and safety problems. Many motor failures are related to excessive heat. A motor can overheat due to a variety of factors including a locked rotor, low line voltage, starting overload, running overload, poor motor ventilation, and/or abnormally high ambient temperatures. Such overheating can damage the insulation on the stator windings, which reduces motor life and can even result in complete motor failure and fires.
Most motors are therefore equipped with thermal overload protection devices that cut off electricity to the motors when motor temperatures exceed a maximum safe level. Many different overload protection devices have been developed including: current sensitive devices such as circuit breakers, fuses, etc. that are typically mounted external to motors and sense current overloads; combination current and temperature sensitive devices such as thermal disc type protectors that are often mounted in motor casings; and temperature sensitive devices that are wired directly to motor windings to directly sense motor winding temperatures.
Governmental and/or regulatory entities have stiffened the efficiency requirements of HVAC systems and other electrical systems with multispeed motors. Such motors typically have two or more sets of stator windings to provide two or more motor speeds. For safety reasons, when independent stator windings are used for different motor speeds, each set must be separately protected against thermal overloads.